Typically, a control valve includes a fluid inlet passage coupled through an orifice to a fluid outlet passage and a closure member disposed in the orifice, which controls the amount of fluid flow therethrough. The closure member may include a valve plug having a surface which engages the orifice forming a valve seat. During operation of a process control system, a process controller may issue a command signal to a control valve to move the valve plug toward or away from engagement with the valve seat to provide a desired fluid flow through the orifice and, therefore, the control valve.
Control valves typically experience variable process conditions during their lifetime. A gas well, for example, may have high pressure when first drilled, but decreasing pressure over time. To maintain constant fluid flow as pressure decreases, the valve must either be disassembled to install a valve seat having a larger flow passage or must be adjusted to allow the valve plug to travel farther from the valve seat to open more port area.
Thus, as a first alternative, to ensure that the valve will achieve the desired flow parameters, separate sets of valve plugs and valve seats may be installed. However, replacement of a valve plug and/or valve seat can necessitate disassembly and reassembly of the valve, which results in additional labor, and additional time the control valve is out of service. Accordingly, it is desirable to be able to achieve a plurality of desired flow parameters by using a single valve plug and a single valve seat.
There are devices which work in conjunction with a valve stem to provide multiple stem travel limits, such as are shown in U.S. Pat. No. 6,905,108. However, the known devices tend to be fairly expensive and difficult to remove and install. This may be due to the tendency to require serrations or threads along the valve stem, as well as along the internal surface of the travel limiting device which engages the valve stem. Also, the devices tend to require removable bolts to fasten pieces together, which may create problems concerning adequate access for manipulation of tools to assemble and disassemble the device during adjustments, as well as having to deal with multiple loose parts which require care in handling so as not to drop them, while at the same time the difficulty of achieving proper thread alignment when installing bolts, etc.
The more difficult and time consuming manipulation involved with the prior art devices can be particularly challenging when an operator is wearing gloves, such as may be necessary in a cold environment. Given that control valves often are directly mounted on a well head and must cycle, making a change with the prior art devices also tends to require that the system be shut down to avoid the normal movement of the valve stem when the control valve cycles. The time required to remove the prior art devices also is important when an operator needs to achieve a more open position, such as to allow the valve to be flushed. Thus, it would be preferable if each maximum flow rate change and flushing operation could be accomplished with less downtime of the control valve, resulting in greater productivity.